1. Field of the Invention
The present invention relates to sorption of various substances from gases, and the fact that the humidity of the gas can impede and adversely affect this said sorption. We have discovered new and improved methods of treating and manufacturing activated carbon in order to convert it to a new, improved form for which the adverse effects of humidity on sorption are eliminated or reduced.
2. Brief Description of the Background
Activated carbon (hereinafter sometimes also referred to as active carbon or as AC) is a highly adsorbent, porous material manufactured from carbonaceous animal, mineral or vegetable matter. The specific surface area of AC is usually several hundred or more square meters per gram, as measured by the BET method. The BET method is named for its originators, Brunauer, Emmet and Teller [J. Amer. Chem. Soc. 60, 309 (1938)] and is explained in the book by J. M. Smith, "Chemical Engineering Kinetics", pp. 329-48, McGraw-Hill Inc., New York (1981). An early disclosure (U.S. Pat. No. 1,497,544 to Chaney) teaches a method of manufacturing AC by subjecting charcoal to differential oxidation to remove exposed hydrocarbons contained therein, followed by limited oxidation to develop high sorption capacity. In said patent Chaney also teaches a method starting with vegetable matter (e.g. coconut shells) and subjecting it first to low-temperature distillation to expel volatile components, then subjecting the residue to differential oxidation (activation) to eliminate exposed hydrocarbons and to develop high sorption capacity. The differential oxidation can be conducted at temperatures in the range of about 800.degree. to 1200.degree. C. using air, oxygen, CO.sub.2, steam or other suitable oxidizing reagents. In U.S. Pat. No. 3,876,505, Stoneburner discloses the manufacture of highly porous activated carbon of high specific surface area from coal by heating 30 min. to 18 hr. in air at a temperature between about 150.degree. C. and 215.degree. C., followed by activating the material in a controlled oxygen atmosphere at a temperature between about 1000.degree. and 2000.degree. F. In a very general way, the manufacture of AC usually involves a pyrolytic carbonization step in which the raw material is heated to convert it to carbon and expel volatile hydrocarbons and other compounds by distillation, followed by an activation step in which the carbon is partly oxidized, thereby converting it to a highly porous structural form that has a large specific surface area. Similarly, AC that has been saturated with an organic sorbent can be regenerated by various combinations of distillation, pyrolysis and activation steps.
Active carbons are highly effective sorbents for treating and removing impurities from gases and liquids, owing to the high porosity and high specific surface area of AC. Said high porosity and surface area are usually produced by an activation process in which tiny pores are burned into the carbons by oxidation with reactants such as steam, air, carbon dioxide or mixtures thereof. Activated carbon usually contains oxygen atoms chemically attached to its surface in the form of functional groups such as the lactones, quinones, phenols and carboxylates reported by Ishizaki and Marti [Carbon 19, 409-12(1981)]. As early as 1863 Smith [Proc. Royal Soc. 12, 424(1863)] found that charcoal binds oxygen chemically. The presence of such chemically bound surface oxygen on AC can originate, at least in part, from the activation process.
It is known that water vapor or humidity in gases, including air, can interfere with the sorption of organic compounds from those gases by activated carbons. Accounts of this adverse effect of humidity on sorption of organic compounds by activated carbon may be found in the following references:
Grant, Joyce & Urbanic, p.219 in "Fundamentals of Adsorption" Proceedings of Engineering Foundation Conference, Deutsche Vereinigung fuer Verfahrenstechnik, 1986.
Walker & Thomson, Naval Research Laboratories Memorandum Report 5791, 20 May 1986.
Nelson, Correia and Harder, Am. Indust. Hygiene Journal, pp. 280-288, May 1976.
Adams, et al, Carbon 26, No. 4, 451-59 (1988).
Generally, sorption of certain molecules by AC is known to be influenced by the chemically bound oxygen on the surface of the carbon. It may be speculated that water vapor in a humid gas adsorbs preferentially on AC by hydrogen bonding at surface oxygen sites on the carbon. It may be speculated further that clusters of water molecules form by hydrogen bonding at such sites and interfere, perhaps by physical blockage, with the adsorption of organic molecules by the carbon.
Another problem with AC arises when it is used to sorb organic substances such as solvents that need to be recovered. During the desorptive recovery process, many such substances degrade or polymerize and it is believed that these undesirable degradation or polymerization reactions are catalyzed by the chemisorbed oxygen groups on the AC. The surface oxygen groups on carbon can be acidic or basic.
A number of authors have discussed the influence of surface-bound oxygen on the sorptive and catalytic properties of activated carbon, including:
Coughlin, I & EC Product R & D 80, 12(1969).
Oda and Yokokawa, Carbon 21, 303-309(1983).
Coughlin, Ezra & Tan, Environ. Sci. & Tech. 2, 291 (1968).
Jankowska et al, Carbon 21, 117-120(1983).
Matsumura et al, Carbon 23, 263-271(1985).
Barton et al, Carbon 22, 265-272(1984).
Boehm and Knoetzinger cited in the following paragraph.
It is known [see for example p 151 of Boehm and Knoetzinger in Catal. Sci. & Tech. (ed. by Anderson & Boudart) 4, 39-207, Springer Verlag, Berlin (1983)] to remove surface oxides from carbons by heating them under vacuum. However, deoxygenating carbons by such outgassing produces a material with a highly reactive and unstable surface that, upon re-exposure to air or moisture or both, will react rapidly to reform surface oxides.
Matsumura et al [Carbon 23, pp 263-271 (1985)] treated activated carbons to remove hydrophilic structures by the following procedure: first, the AC was washed with concentrated hydrochloric acid and then with concentrated hydrofluoric acid to remove metallic ions and then thoroughly rinsed with water. It is likely that contacting the carbon with hydrofluoric acid caused chemical reaction between the surface oxygen groups on the carbon and the said acid, thereby producing additional changes in the carbon beyond the removal of metals. Subsequently, a 30-g sample was put in a quartz cell, evacuated and heated in a furnace at 1000.degree. C. for 30 min, then put into contact with hydrogen of about 500 torr at 1000.degree. C., and again evacuated. This treatment process was repeated a few times, and finally the product material was cooled slowly to room temperature in the hydrogen atmosphere. Matsumura et al found that their treatment decreased the adsorption affinities of their treated AC for methanol and water, but not for benzene. They did not study the influence of humidity upon sorption of organic molecules on AC treated by their method. Surprisingly, and in spite of the procedure described by Matsumura et al, we have found that their expensive, inconvenient and time-consuming steps of treatment with concentrated mineral acids to remove metal ions and then washing, are not necessary to desensitize activated carbons to the deleterious effects of humidity upon sorption of organic molecules. In fact, by our methods disclosed hereinbelow, we have been able successfully to reduce the adverse effects of humidity upon sorption of organic substances by active carbons which contain substantial concentrations of metal ions. Contrary to any implications of the teaching of Matsumura et al, we have not had to remove said metals in order to desensitize active carbons to the deleterious effects of humidity upon sorption of organic compounds.
Mazur et al [J. Am. Chem. Soc. 99, pp 3888-91 (1977)] freed synthetic carbon fibers of surface oxides by pyrolysis at 1020.degree. C. in a vacuum of 10.sup.-5 torr, a procedure called vacuum-outgassing. After cooling to room temperature, Mazur et al exposed their vacuum-outgassed samples of carbon fibers to vapors of a variety of substances including oxygen, propane, ethylene, propylene, isobutylene, allene, cyclopentadiene, methylacrylate, acrylyl chloride and vinyl bromide. All of these substances were found to adsorb irreversibly on the deoxygenated synthetic carbon fibers. Mazur et al did not work with activated carbons and did not investigate sorption.
Attar discloses in U.S. Pat. No. 4,597,769 a process for demineralizing coal in which the surface forces between organic and inorganic phases of the coal were reduced by exposing the coal to a mixture of alcohol and acidic gas at temperatures between 100.degree. and 250.degree. C. for up to 300 min.
It appears that the prior art does not teach the manufacture or preparation of activated carbons that resist the deleterious effect of humidity upon the sorption of organic substances. Neither does the prior art teach how to treat activated carbons to make them so resistant.
The prior art described above does not disclose any improved active carbon sorbent resistant to the deleterious effects of humidity on the sorption of organic molecules, nor teach any method of making such an improved activated-carbon sorbent. In spite of the above described prior art, there remains a need for improved activated carbon sorbents resistant to the adverse effects of humidity.